Thread: Does reducing the powder charge for a 12 gauge slug safely reduce projectile speed?

A friend I have loaded some 12 gauge slugs in the past has asked me to reduce the fps by about 400 fps.
Being a noobee, I assiduously followed the manufacturer's loading books instructions and am uncertain how to achieve this nor do I fully comprehend all the unfortunate side effects which may occur.

These are both sabot slugs (using a WAA 12 wad with trimmed petals.) Lyman 525 gr and Lee 1 oz which I have loaded according to these manufacturer's recommendations, being:

Lyman . Lymans 5 th edition Shotshell Reloading handbook page 380....28 grains AP70N 1400 fps
Lee. Loading instructions supplied with their Lee 1 oz slug mould.....29 grains AP70N 1450 fps

Please, there are three issues upon which I seek your advice / opinions...........and of course any extras you may feel necessary.

1...Will just reducing the powder load result in a slower slug speed? I would suspect so, but by how many grains to achieve the 400 fps speed reduction? I can of course do a range of powder loads but am uncertain of the range variation, i e three loads, starting first at 13 grains, then increase each by 3 grains
I realise that this is probably going to be subsonic but think that although not being told so, this is what he wants.

2...Will reducing the powder load be an invitation for severe loss of projectile speed and control? I would expect so if too much of a powder reduction was made. Is that inevitable when any reduction is made or is there a lower threshold before it becomes troublesome?
3...Apart from the possibility of a reduced powder load not completely emptying the barrel and leaving an obstruction within, what other are undesirable side effects could arise?

Your advice would be appreciated.